1. Field of the Invention
The present invention relates to sensor devices, and more particularly, potentiometric sensors comprising an internal reference element, a membrane, and a fortiophore. The fortiophore, an electrochemical agent, interfaces the membrane and the internal reference element to form a solid internal contact.
2. Technical Review
Conventional sensors (3) known in the art (FIG. 1) have a layer of metal, e.g. Ag or other electrically conductive material, a layer of metal halide, e.g. AgCl, an aqueous or dry internal filling solution (usually containing the chloride salt of the cation being analyzed, e.g. KCl), and an ion selective membrane. See generally Working with Ion-Selective Electrodes, Camman, K. Springer-Verlag, 1979. The ion selective membrane includes an ionophore. See Ammann et al. Helvetica ACTA 1975, 58, 1535-1548. An ionophore is an ion-selective compound which is permselective, e.g. capable of complexing a desired ion and extracting it without a counterion into the interfacial zone of the membrane. The internal filling solution can form electrochemically defined interfaces with the metal halide on one side and the ionophore doped in the membrane. Such internal filling solutions actually contain a constant activity of the communicating ions which provide a high and dominating exchange current at both interfaces and therefore constant and predictable potentials at both the metal halide and the inner surface of the membrane. The outer surface of the membrane is exposed to the test sample. The potential generated at this interface is, according to the Nernst equation, dependant on the activity of the test sample ion, which the membrane is selective for.
Coated wire electrodes (CWE) have a layer of Ag or other conductive material, an optional layer of AgCl, and an ion selective membrane. See generally chapter four of Principles of Chemical Sensors, Janata, J., Plenum Press, 1989. There is no internal fill solution to interface the AgCl with the membrane to maintain the constant potential as in the conventional electrodes. The potential is therefore determined by unknown interfacial charge exchange agents or sites of uncontrolled activities. Potential measurements for the CWE tend to drift, have a slow response time, and have unreproducible potential offsets due to the undefined interface between the membrane and electrochemical internal reference element. In addition, any minute amounts of water soluable salt at the interface will cause water uptake causing drift in potential.
In attempting to reduce an ion selective sensor to a miniaturized planar configuration, problems arise due to differences in storage and measuring conditions. Water will permeate the membrane at any time to maintain osmotic balance. If water permeates from the sample into the internal electrolyte, the membrane will bulge or it may burst. If water leaves (e.g. evaporation) the sensor, the membrane will crenate. Moreover, this would change the activity of the internal electrolyte solution, thereby causing potential drift. This relationship is not considered in larger conventional electrodes because of the large reservoir of internal fill solution, but in a small sensor such as made by planar processing technologies, it becomes critical.
Shono et al., U.S. Pat. Nos. 4,554,362 and 4,523,994, describe the use of bis-crown-ether derivatives as neutral carriers in ion-selective membranes of ion-selective electrodes.
Delton et al., U.S. Pat. No. 4,504,368, describe the use of crown-ether compositions as ionophores in ion-selective compositions and electrodes. Various solvents are disclosed to solvate the crown ether and to provide ion mobility in the membrane.
Battaglia et al., U.S. Pat. No. 4,214,968, describe dry-operative ion selective electrodes incorporating the use of ionophores.
Freiser et al., U.S. Pat. No. 4,115,209 describe an electrode formed by coating a conductive substrate with an ion exchange material in a matrix. A listing of potentiometrically measurable ion or group of ions is provided.
Baginski et al., EP 0 267 724, disclose a method of printing an electrochemically active material on a substrate to provide a test device for carrying out a microchemical test.
Oue et al., Chem. Ltr. 1988, 409-410 disclose the use of monothiacrown ether (MTCE) as a neutral carrier for Ag-selective electrodes. It is noted that Que in a letter dated Sep. 6, 1988 recommended that if the compound is used as a neutral silver ion carrier, it should be first complexed with AgNO.sub.3 in order to reduce conditioning time.
Daunert et al., Anal. Chem. 1990, 62, 1428-1431 describe ion-selective electrodes including an ionophore covalently attached to a polymeric matrix.
Oue, M. et al., J. Chem. Soc.-Perkin Trans. 1989, 1675-1678 disclose the use of lipophilic mono-and di-thiacrown ethers as neutral carriers of polymeric membrane Ag.sup.+ -selective electrodes.